This invention relates to acetal polymers having improved thermal stability.
Molding compositions comprising acetal polymers have been in commercial use for many years. They have application in a wide variety of end uses, e.g. automobile applications such as bumper extensions and instrument panels, plumbing supplies such as valves, shower assemblies, flush tank components, faucets and pipe fittings, tool components such as screw driver adaptors, and household and personal products, such as quick boiling electric water kettles, clothes hangers and combs.
While acetal polymers have a highly favorable spectrum of physical properties which are responsible for their wide commercial acceptance, the unmodified polymer tends to suffer some thermal and oxidative degradation when subjected to elevated temperatures, e.g., over 200.degree. C. Thus various expedients have been proposed for minimizing this problem, including in the product area, the addition of phenolic anti-oxidants as disclosed in U.S. Pat. Nos. 3,103,499 and 3,240,753 and the addition of amidine compounds alone or in conjunction with amino substituted amides as thermal stabilizers for the polymer as taught by U.S. Pat. Nos. 3,313,767 and 3,314,918. The stabilizers are generally considered to function as formaldehyde and acid scavengers. Process expedients for the improvement of the thermal properties of the polymer include various types of hydrolysis designed for acetal copolymers containing carbon to carbon bonds in the polymer chain, e.g. oxymethylene-oxyethylene copolymers, as disclosed, for example in U.S. Pat. Nos. 3,174,948; 3,219,623; 3,318,848; and 3,418,280, and the endcapping of hemiformal groups of oxymethylene homopolymers by acylation or etherification, e.g. as disclosed in U.S. Pat. No. 3.170,896.
Two of the most effective amidine stabilizers used in acetal polymer compositions are cyanoguanidine and melamine. However, although the employment of either of these stabilizers, particularly in combination with the other expedients set out previously, yields acetal polymer compositions satisfactory for many applications, there exist other applications for which an even greater degree of thermal stability is desirable. Moreover, any expedient having the effect of increasing the efficacy of the stabilizer so that a desired degree of stabilization can be achieved with a reduced amount of stabilizer is considered highly advantageous.
Also known in the art as disclosed in U.S. Pat. No. 4,098,843 of Johnson is the predispersion of any of certain superpolyamide stabilizers in a carrier resin, which is subsequently admixed with an acetal polymer. The cited patent states that the use of a predispersion of the polyamide has the effect of reducing mold deposits during the molding of the acetal polymer.
U.S. Pat. No. 4,351,916 of Kohan discloses a variation of the process described in U.S. Pat. No. 4,098,843, wherein both the polyamide stabilizer and a poly(ethylene glycol), added to reduce melt viscosity, are predispersed in a carrier resin which is then admixed with the acetal polymer. It is stated by the patentee that in this process, the effect of the predispersion is to reduce the destabilizing effect caused by impurities in the poly(ethylene glycol). The patentee also states that "U.S. Pat. No. 4,098,843 shows no stabilizing advantage over U.S. Pat. No. 2,993,025 and, indeed, carries the implication of larger additive requirements for comparable stability because of the presence of the carrier resin." (column 1, lines 36-40). The second-cited patent in the foregoing quotation, U.S. Pat. No. 2,993,025, contains the original disclosure of the use of polyamides as stabilizers for acetal polymers (polyoxymethylenes).
Pending application Ser. No. 644,796 filed Oct. 25, 1984, now U.S. Pat. No. 4,582,405, by A. B. Auerbach and K. M. Natarajan discloses a method of reducing black speck formation caused by compounding acetal polymers and polyamides having a melting or softening point below that of the acetal polymer by adding the polyamide to the acetal polymer as a dispersion in a carrier resin.